The effect of lipid composition and structure on the lipoprotein lipase-catalyzed hydrolysis of triacylglycerols was determined in a monolayer system consisting of purified bovine milk lipoprotein lipase and fatty acid-free albumin. In a monolayer of dioleoylphosphatidylcholine containing 1-6 mol% of either tri[14C]oleoylglycerol or tri[14C] palmitoylglycerol , lipoprotein lipase catalyzed the hydrolysis of the unsaturated triacylglycerol at a higher rate than the saturated lipid and in either the presence or absence of apolipoprotein C-II, the activator protein for the enzyme. For example, with 3 mol% triacylglycerol and in the presence of apolipoprotein C-II, the rate of the lipoprotein lipase-catalyzed hydrolysis of tri[14C]oleoylglycerol was 27 mumol oleic acid produced/h per mg enzyme vs. 12 mumol for tri[14C] palmitoylglycerol . The effect of phospholipid fatty acyl chain length and unsaturation/saturation, polar head group and surface density on the lipoprotein lipase-catalyzed hydrolysis of tri[14C]oleoylglycerol was determined. The rate of enzyme hydrolysis of triacylglycerol was similar whether the phospholipid was a diester or diether lipid or the polar head group was ethanolamine or choline. In general, phospholipids with shorter and unsaturated fatty acyl chains gave higher rates of lipoprotein lipase hydrolysis of triacylglycerol than the corresponding longer and saturated lipids. However, with all phospholipids tested, the rate of enzyme hydrolysis decreased with increasing surface density. Lipoprotein lipase showed no activity toward triacylglycerol in a monolayer of sphingomyelin; addition of dioleoylphosphatidylcholine to the monolayer enhanced the rate of enzyme catalysis. Cholesterol (50 mol%) in a dipalmitoylphosphatidylcholine monolayer increased the rate of the lipoprotein lipase-catalyzed hydrolysis of tri[14C]oleoylglycerol, whereas cholesterol decreased the rate in a dioleoylphosphatidylcholine monolayer. The effect of phospholipid structure and surface density on lipoprotein lipase activity could not be accounted for by the amount of apolipoprotein C-II which was present at the interface. Based on these findings and other reports in the literature, we suggest that the catalytic activity of lipoprotein lipase toward tri[14C] oleylglycerol in various monolayers is dependent on the conformation or appropriate physical state of the triacylglycerol substrate at the lipid interface.